This invention relates generally to drill bits used to drill boreholes in subterranean formations and, more particularly, to a method and apparatus for balancing the drill bits.
In the exploration of oil, gas and geothermal energy, drilling operations are used to create boreholes in the earth. One type of drilling operation includes rotary drilling. According to rotary drilling, the borehole is created by rotating a tubular drill string which has a drill bit coupled to one end. As the drill bit rotates and deepens the borehole, additional drill strings are coupled to the end that does not have the drill bit so that the drill bit may further deepen the borehole. As the drill bit rotates and cuts through the formation, the drill bit becomes hot and creates debris in the form of rock cuttings. A drilling fluid may be pumped through the center of the hollow drill string so that the drill bit may be cooled and lubricated and the debris may be carried away. The drilling fluid travels through the drill string and exits the drill bit at increased velocity through one or more nozzles on the drill bit's outer surface. The drilling fluid then returns to the surface via an annular space which is created between the inner surface of the borehole and the outer surface of the drill string.
One type of bit used for rotary drilling is a drag bit or a fixed cutter bit. These drag bits have a plurality of blades that have a plurality of cutters attached to each of the blades. As the drag bit is rotated, the cutters scrape against the bottom and sides of the borehole to cut away rock. As the rate of penetration of the drill increases, the effective life of these drag bits are substantially decreased because the cutters become cracked and occasionally are violently torn from the blade.
A substantial portion of these destructive forces are caused by radial imbalance forces, which are the forces occurring perpendicular to the longitudinal axis of the drill bit. These radial imbalance forces cause the drill bit to rotate about a center offset from the geometric center of the bit body, or geometric bit axis, in such a way that the drill bit tends to backwards whirl about the borehole or to enlarge the borehole from the nominal diameter. The true rotational axis of the drill bit is most likely not the geometric axis of the drill bit. This backwards whirl causes the center of rotation to change dynamically. Thus, the cutters become exposed to greatly increased impact loads or higher change in the cutter loading during one revolution of the bit, thereby destroying the cutters.
The use of blade asymmetry in full hole PDC drill bits is common as an anti-whirl configuration. Blade asymmetry in almost all cases will shift the mass center of the bit off of the geometric center of the bit. The mass center shifting off of the geometric center of the bit results in additional imbalance forces on the drill bit and also contributes to destroying the cutters.
Time and money is consumed when cutters are destroyed. The drilling process stops, the drill string must be removed, a new drill bit must be attached to the drill string, and the time period to obtain the profitable oil, gas, and/or geothermal energy is delayed. Thus, manufactures attempt to force balance the bit so that the rotational axis of the bit is the same as the drill string center, or geometric bit axis. The force is typically balanced by setting the cutters so that the resultant radial imbalance force is zero, which is the sum of all centrifugal forces and the sum of all the centripetal forces, according to the best case scenario. Additionally, the imbalance ratio, which is the ratio of the resultant radial imbalance force to the weight-on-bit (WOB) force, should be within a certain desired value according to manufacturer criteria. According to some manufacturers, the imbalance ratio should be 10% or less. Current force balancing techniques do not take into account any shift in the mass center of the bit off of the geometric center.
An additional concern arises when attempting to force balance bi-center bits, which have a reamer section and a pilot section. The pilot section is usually smaller and is coupled to the reamer section. The pilot section has its pilot section geometric center, while the reamer section has its reamer section geometric center. Since the bi-center bit typically has two centers, the radial imbalance forces are balanced on the pilot section geometric center, not both of them. Despite these force balancing efforts, the life of the cutters may be further increased by using more innovative balancing techniques.
U.S. Pat. No. 5,010,789 (the “'789 Patent”), issued to Brett et al. on Apr. 30, 1991, discloses a method of making imbalanced compensated drill bits. The teachings disclosed in the '789 Patent are incorporated by reference herein.
In view of the foregoing discussion, need is apparent in the art for improving the drill bits so that the life of the cutters are increased. Additionally, a need exists for improving methods for balancing the drill bit. Furthermore, a need exists for improved force balancing methods that account for the shift in the mass center of the bit. Moreover, there exists a need to produce an improved forced balanced bit that has blade asymmetry. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit down hole drilling, for example creating boreholes more effectively and more profitably. This technology is included within the current invention.